Processing of semiconductor components with dense processing fluids and ultrasonic energy

ABSTRACT

Method for processing an article with a dense processing fluid in a processing chamber while applying ultrasonic energy during processing. The dense fluid may be generated in a separate pressurization vessel and transferred to the processing chamber, or alternatively may be generated directly in the processing chamber. A processing agent may be added to the pressurization vessel, to the processing chamber, or to the dense fluid during transfer from the pressurization vessel to the processing chamber. The ultrasonic energy may be generated continuously at a constant frequency or at variable frequencies. Alternatively, the ultrasonic energy may be generated intermittently.

BACKGROUND OF THE INVENTION

[0001] Small quantities of contamination are detrimental to themicrochip fabrication process in the manufacturing of semiconductorelectronic components. Contamination in the form of particulates, films,or molecules causes short circuits, open circuits, silicon crystalstacking faults, and other defects. These defects can cause the failureof finished microelectronic circuits, and such failures causesignificant yield reductions, which greatly increases manufacturingcosts.

[0002] Microelectronic circuit fabrication requires many processingsteps. Processing is performed under extremely clean conditions and theamount of contamination needed to cause fatal defects in microcircuitsis extremely small. For example, an individual particle as small as 0.01micrometer in size can result in a killer defect in a modernmicrocircuit. Microcontamination may occur at any time during the manysteps needed to complete the microcircuit. Therefore, periodic cleaningof the wafers used for microelectronic circuits is needed to maintaineconomical yields. Also, tight control of purity and cleanliness of theprocessing materials is required.

[0003] Numerous cleaning methods have been used in the manufacture ofsemiconductor electronic components. These include immersion in liquidcleaning agents to remove contamination through dissolution and chemicalreaction. Such immersion may also serve to reduce the van der Waalsadhesive forces and introduce double layer repulsion forces, therebypromoting the release of insoluble particles from surfaces. A standardwet cleaning process in common use begins with exposure to a mixture ofH₂SO₄, H₂O₂, and H₂O at 110-130° C., and is followed by immersion in HFor dilute HF at 20-25° C. Next a mixture of NH₄OH, H₂O₂, and H₂O at60-80° C. removes particles, and a mixture of HC₁, H₂O₂, and H₂O at60-80° C. removes metal contamination. Each of these steps is followedby a high purity H₂O rinse. This wet cleaning process reachesfundamental barriers at dimensions less than 0.10 micrometer. As thedevice geometries shrink and gate oxide thickness decreases,sub-micrometer particle removal becomes increasingly difficult.

[0004] Stripping/removal of primarily organic photoresist may beperformed using dilute aqueous mixtures containing H₂SO₄ and H₂O₂.Alternatively, the stripping/removal may be performed using a two-stepplasma, or reactive ion etching process, followed by wet chemicalcleaning of the residue material. Ozonated H₂O has been used for thedecomposition of hydrocarbon surface contaminants on silicon wafers.

[0005] Brush scrubbing has been used to enhance the liquid immersionprocess by introducing hydrodynamic shear forces to the contaminatedsurfaces. A typical application uses a wafer cleaning apparatuscomprising two opposed brushes for brushing a vertically disposed waferin a tank that can contain a process liquid.

[0006] The addition of ultrasonic energy can increase the effectivenessof the liquid immersion process. Sound waves vibrating at frequenciesgreater than 20,000 cycles per second (20 KHz), i.e., beyond the rangeof human hearing, have been used to transmit high frequency energy intoliquid cleaning solutions.

[0007] Wet processing methods may become problematic as microelectroniccircuit dimensions decrease and as environmental restrictions increase.Among the limitations of wet processing are the progressivecontamination of re-circulated liquids, re-deposition from contaminatedchemicals, special disposal requirements, environmental damage, specialsafety procedures during handling, reduced effectiveness in deeplypatterned surfaces due to surface tension effects and image collapse(topography sensitivity), dependence of cleaning effectiveness onsurface wet-ability to prevent re-adhesion of contaminants, and possibleliquid residue causing adhesion of remaining particles. Aqueous cleaningagents that depend upon chemical reaction with surface contaminants mayalso present compatibility problems with new thin film materials, orwith more corrosion-prone metals such as copper. In addition, theInternational Technology Roadmap for Semiconductors has recommended a62% reduction in water use by the year 2005 and an 84% reduction by theyear 2014 to prevent water shortages. With the continuing trend towardincreasing wafer diameters having a larger precision surface area,larger volumes of liquid chemicals will be required in the fabricationprocess.

[0008] In view of these problems, methods for dry (anhydrous) surfacecleaning of semiconductor electronic components are being developed.Among these are gas jet cleaning to remove relatively large particlesfrom silicon wafers. However, gas jets can be ineffective for removingparticles smaller than about 5 micrometers in diameter because theforces that hold particles on the surface are proportional to theparticle size, while the aerodynamic drag forces generated by theflowing gas for removing the particles are proportional to the particlediameter squared. Therefore, the ratio of these forces tends to favoradhesion as the particle size shrinks. In addition, smaller particlesare not exposed to strong drag forces in the jet since they normally liewithin the surface boundary layer where the gas velocity is low.

[0009] Exposure to ozone combined with ultraviolet light can be used todecompose contaminating hydrocarbons from surfaces, but this techniquehas not been shown to remove inorganic contaminants or particleseffectively.

[0010] Other alternatives to wet cleaning include the use of jetscontaining snow or pellet projectiles comprising frozen Ar, N₂, H₂O orCO₂ which are used to “sandblast” contaminated surfaces. In theseprocesses, pressurized gaseous or gas/liquid mixtures are expanded in anozzle to a pressure near or below atmospheric pressure. The resultingJoule-Thomson cooling forms solid or liquid aerosol particles, whichtraverse the boundary layer and strike the contaminated surface. Thistechnique requires extremely clean and pure processing materials. Tracemolecular contaminants (e.g., hydrocarbons) in the feed gases cancondense into solid particulates or droplets upon expansion, causingdeposition of new contaminants on the surface. Although useful inproviding removal of many surface contaminants, these processes cannotremove all of the important contaminants present on a wafer surface, andhave not yet found wide acceptance in the semiconductor industry.

[0011] Immersion in supercritical fluids is another alternative to wetcleaning. The effectiveness of supercritical fluids in various cleaningand extraction applications is well established and extensivelydocumented. Supercritical fluids have solvent power much greater thanthe corresponding gaseous state, and can effectively dissolve and removeunwanted films and molecular contaminants from a precision surface. Thecontaminants can be separated from the cleaning agent by a reduction inpressure below the critical value, which concentrates the contaminantsfor disposal and permits recovery and re-use of the cleaning fluid.

[0012] Supercritical CO₂ in particular has been used as a versatile andcost effective method to overcome the above-mentioned problems in wafercleaning. Supercritical CO₂ effectively cleans parts with increasinglysmaller dimensions and lowers water usage, thereby yielding improvementsin performance and environmental benefits. Preliminary Cost of Ownership(CoO) studies have shown that supercritical CO₂ cleaning is also morecost effective when compared to aqueous cleaning. CO₂ in thesupercritical state has particularly good solvent properties and hasbeen found to be effective in removing organic impurities. It can bemodified with added co-solvents or entrainers to widen the range ofcontaminants that can be removed, including particles, native orchemical oxides, metallic contaminants, and other inorganic materials.Ultrasonic energy can be introduced into supercritical fluid cleaningreactors to enhance the efficiency of the cleaning process.

[0013] Future microcircuits will have smaller feature sizes and greatercomplexities, and will require more processing steps in theirfabrication. Contamination control in the process materials systems andprocessing environment will become even more critical. In view of theseanticipated developments, there is a need for improved wafer cleaningmethods to maintain or improve economical yields in the manufacture ofthese smaller and more complex microelectronic systems. In addition, theadvent of smaller feature sizes and greater complexities will requireimproved fabrication processes steps including etching, thin filmdeposition, planarization, and photoresist development. Embodiments ofthe present invention, which are described below and defined by thefollowing claims, address this need by improved processing methodsutilizing dense processing fluids with the application of ultrasonicenergy.

BRIEF SUMMARY OF THE INVENTION

[0014] A first embodiment of the invention includes a method forprocessing an article comprising:

[0015] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0016] (b) preparing a dense fluid by:

[0017] (b1) introducing a subcritical fluid into a pressurization vesseland isolating the vessel; and

[0018] (b2) heating the subcritical fluid at essentially constant volumeand essentially constant density to yield a dense fluid;

[0019] (c) transferring at least a portion of the dense fluid from thepressurization vessel to the processing chamber, wherein the transfer ofthe dense fluid is driven by the difference between the pressure in thepressurization vessel and the pressure in the processing chamber,thereby pressurizing the processing chamber with transferred densefluid;

[0020] (d) introducing one or more processing agents into the processingchamber either before (c), or during (c), or after (c) to provide adense processing fluid;

[0021] (e) introducing ultrasonic energy into the processing chamber andcontacting the article with the dense processing fluid to yield a spentdense processing fluid and a treated article; and

[0022] (f) separating the spent dense processing fluid from the treatedarticle.

[0023] The dense fluid may be generated in (b2) at a reduced temperaturein the pressurization vessel below about 1.8, wherein the reducedtemperature is defined as the average absolute temperature of the densefluid in the pressurization vessel after heating divided by the absolutecritical temperature of the fluid. The contacting of the article withthe dense processing fluid in the processing chamber in (d) may beeffected at a reduced temperature in the processing chamber betweenabout 0.8 and about 1.8, wherein the reduced temperature is defined asthe average absolute temperature of the dense processing fluid in theprocessing chamber during (d) divided by the absolute criticaltemperature of the dense processing fluid.

[0024] The dense fluid may comprise one or more components selected fromthe group consisting of carbon dioxide, nitrogen, methane, oxygen,ozone, argon, hydrogen, helium, ammonia, nitrous oxide, hydrogenfluoride, hydrogen chloride, sulfur trioxide, sulfur hexafluoride,nitrogen trifluoride, monofluoromethane, difluoromethane,trifluoromethane, trifluoroethane, tetrafluoroethane, pentafluoroethane,perfluoropropane, pentafluoropropane, hexafluoroethane,hexafluoropropylene, hexafluorobutadiene, and octafluorocyclobutane andtetrafluorochloroethane. The dense fluid may comprise one or morehydrocarbons having 2 to 6 carbon atoms.

[0025] The total concentration of the one or more processing agents inthe dense processing fluid may be between about 0.1 and 20 wt %. In oneembodiment, the dense processing fluid may comprise one or moreprocessing agents selected from the group consisting of ethyl acetate,ethyl lactate, propyl acetate, butyl acetate, diethyl ether, dipropylether, methanol, ethanol, isopropanol, acetonitrile, propionitrile,benzonitrile, ethylene cyanohydrin, ethylene glycol, propylene glycol,ethylene glycol monoacetate, propylene glycol monoacetate, acetone,butanone, acetophenone, trifluoroacetophenone, triethyl amine, tripropylamine, tributyl amine, 2,4, dimethyl pyridine, dimethylethanolamine,diethylethanolamine, diethylmethanolamine, dimethylmethanolamine,dimethylformamide, dimethylacetamide, ethylene carbonate, propylenecarbonate, acetic acid, lactic acid, butane-diol, propane-diol,n-hexane, n-butane, hydrogen peroxide, t-butyl hydroperoxide,ethylenediaminetetraacetic acid, catechol, choline, and trifluoroaceticanhydride.

[0026] In another embodiment, the dense processing fluid may compriseone or more processing agents selected from the group consisting ofhydrogen fluoride, hydrogen chloride, chlorine trifluoride, nitrogentrifluoride, monofluoromethane, difluoromethane, trifluoromethane,trifluoroethane, tetrafluoroethane, pentafluoroethane, perfluoropropane,pentafluoropropane, hexafluoroethane, hexafluoropropylene,hexafluorobutadiene, octafluorocyclobutane tetrafluorochloroethane,fluoroxytrifluoromethane (CF₄O), bis(difluoroxy)methane (CF₄O₂),cyanuric fluoride (C₃F₃N₃), oxalyl fluoride (C₂F₂O₂), nitrosyl fluoride(FNO), carbonyl fluoride (CF₂O), and perfluoromethylamine (CF₅N).

[0027] In yet another embodiment, the dense processing fluid maycomprise one or more processing agents selected from the groupconsisting of organometallic precursors, photoresists, photoresistdevelopers, interlayer dielectric materials, silane reagents, andstain-resistant coatings.

[0028] The pressure of the spent dense processing fluid may be reducedto yield at least a fluid phase and a residual compound phase, and thephases may be separated to yield a purified fluid and recovered residualcompounds. The purified fluid may be recycled to provide a portion ofthe subcritical fluid in (b1). The pressure of the purified fluid may bereduced to yield a further-purified fluid phase and an additionalresidual compound phase, and the phases may be separated to yield afurther-purified fluid and additional recovered residual compounds. Thefurther-purified fluid may be recycled to provide a portion of thesubcritical fluid in (b1).

[0029] The subcritical fluid in the pressurization vessel prior toheating in (b2) may comprise a vapor phase, a liquid phase, orcoexisting vapor and liquid phases.

[0030] Another embodiment of the invention includes a method forprocessing an article comprising:

[0031] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0032] (b) preparing a dense processing fluid by:

[0033] (b1) introducing a subcritical fluid into a pressurization vesseland isolating the vessel;

[0034] (b2) heating the subcritical fluid at essentially constant volumeand essentially constant density to yield a dense fluid; and

[0035] (b3) introducing one or more processing agents into thepressurization vessel

[0036] before introducing the subcritical fluid into the pressurizationvessel, or

[0037] after introducing the subcritical fluid into the pressurizationvessel but before heating the pressurization vessel, or

[0038] after introducing the subcritical fluid into the pressurizationvessel and after heating the pressurization vessel;

[0039] (c) transferring at least a portion of the dense processing fluidfrom the pressurization vessel to the processing chamber, wherein thetransfer of the dense processing fluid is driven by the differencebetween the pressure in the pressurization vessel and the pressure inthe processing chamber, thereby pressurizing the processing chamber withtransferred dense processing fluid;

[0040] (d) introducing ultrasonic energy into the processing chamber andcontacting the article with the transferred dense processing fluid toyield a spent dense processing fluid and a treated article; and

[0041] (e) separating the spent dense processing fluid from the treatedarticle.

[0042] A further embodiment of the invention includes an apparatus forprocessing an article which comprises:

[0043] (a) a fluid storage tank containing a subcritical fluid;

[0044] (b) one or more pressurization vessels and piping means fortransferring the subcritical fluid from the fluid storage tank to one ormore pressurization vessels;

[0045] (c) heating means to heat the contents of each of the one or morepressurization vessels at essentially constant volume and essentiallyconstant density to convert the subcritical fluid into a dense fluid;

[0046] (d) a sealable processing chamber for contacting an article withthe dense fluid;

[0047] (e) ultrasonic generation means for introducing ultrasonic energyinto the sealable processing chamber;

[0048] (f) piping means for transferring the dense fluid from the one ormore pressurization vessels into the sealable processing chamber; and

[0049] (g) one or more processing agent storage vessels and pumpingmeans to inject one or more processing agents (1) into the one or morepressurization vessels or (2) into the piping means for transferring thedense fluid from the one or more pressurization vessels to the sealableprocessing chamber or (3) into the sealable processing chamber.

[0050] The apparatus may further comprise pressure reduction means andphase separation means to separate a spent dense processing fluidwithdrawn from the processing chamber to yield at least a purified fluidand one or more recovered residual compounds. The apparatus may furthercomprise recycle means to recycle the purified fluid to the fluidstorage tank.

[0051] Another embodiment of the invention relates to a method forprocessing an article comprising:

[0052] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0053] (b) providing a dense processing fluid in the processing chamber;

[0054] (c) introducing ultrasonic energy into the processing chamber andvarying the frequency of the ultrasonic energy while contacting thearticle with the dense processing fluid to yield a spent denseprocessing fluid and a treated article; and

[0055] (e) separating the spent dense processing fluid from the treatedarticle. The frequency of the ultrasonic energy may be increased during(c). Alternatively, the method of claim 19 wherein the frequency of theultrasonic energy may be decreased during (c).

[0056] The dense processing fluid may be prepared by:

[0057] (a) introducing a subcritical fluid into a pressurization vesseland isolating the vessel;

[0058] (b) heating the subcritical fluid at essentially constant volumeand essentially constant density to yield a dense fluid; and

[0059] (c) providing the dense processing fluid by one or more stepsselected from the group consisting of

[0060] (1) introducing one or more processing agents into the densefluid while transferring the dense fluid from the pressurization vesselto the processing chamber,

[0061] (2) introducing one or more processing agents into thepressurization vessel to form a dense processing fluid and transferringthe dense processing fluid from the pressurization vessel to theprocessing chamber,

[0062] (3) introducing one or more processing agents into the densefluid in the processing chamber after transferring the dense fluid fromthe pressurization vessel to the processing chamber,

[0063] (4) introducing one or more processing agents into thepressurization vessel before introducing the subcritical fluid into thepressurization vessel,

[0064] (5) introducing one or more processing agents into thepressurization vessel after introducing the subcritical fluid into thepressurization vessel but before heating the pressurization vessel, and

[0065] (6) introducing one or more processing agents into thepressurization vessel after introducing the subcritical fluid into thepressurization vessel and after heating the pressurization vessel.

[0066] Alternatively, the dense processing fluid may be prepared by:

[0067] (a) introducing a subcritical fluid into the sealable processingchamber and isolating the chamber;

[0068] (b) heating the subcritical fluid at essentially constant volumeand essentially constant density to yield a dense fluid; and

[0069] (c) providing the dense processing fluid by one or more stepsselected from the group consisting of

[0070] (1) introducing one or more processing agents into the sealableprocessing chamber before introducing the subcritical fluid into thesealable processing chamber,

[0071] (2) introducing one or more processing agents into the sealableprocessing chamber after introducing the subcritical fluid into thesealable processing chamber but before heating the subcritical fluidtherein, and

[0072] (3) introducing one or more processing agents into the sealableprocessing chamber after introducing the subcritical fluid into thesealable processing chamber and after heating the subcritical fluidtherein.

[0073] In an alternative embodiment of the invention, an article may beprocessed by a method comprising:

[0074] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0075] (b) providing a dense fluid in the processing chamber;

[0076] (c) introducing ultrasonic energy into the processing chamber andvarying the frequency of the ultrasonic energy while contacting thearticle with the dense fluid to yield a spent dense fluid and a treatedarticle; and

[0077] (e) separating the spent dense fluid from the treated article.

[0078] Another embodiment of the invention may include a method forprocessing an article comprising:

[0079] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0080] (b) providing a dense processing fluid in the processing chamber;

[0081] (c) introducing ultrasonic energy into the processing chamberintermittently while contacting the article with the dense processingfluid to yield a spent dense processing fluid and a treated article; and

[0082] (e) separating the spent dense processing fluid from the treatedarticle.

[0083] In a further embodiment, a method for processing an article maycomprise:

[0084] (a) introducing the article into a sealable processing chamberand sealing the processing chamber;

[0085] (b) providing a dense fluid in the processing chamber;

[0086] (c) introducing ultrasonic energy into the processing chamberintermittently while contacting the article with the dense fluid toyield a spent dense fluid and a treated article; and

[0087] (e) separating the spent dense fluid from the treated article.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0088]FIG. 1 is a density-temperature phase diagram for carbon dioxide.

[0089]FIG. 2 is a generalized density-temperature phase diagram.

[0090]FIG. 3 is a process flow diagram illustrating an embodiment of theinvention.

[0091]FIG. 4 is a schematic drawing of a pressurization vessel used inthe embodiment of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0092] Cleaning is the most frequently repeated step in the manufactureof integrated circuits. At the 0.18-micrometer design rule, 80 of theapproximately 400 total processing steps are cleaning steps. Waferstypically are cleaned after every contaminating process step and beforeeach high temperature operation to ensure the quality of the circuit.Exemplary cleaning and removal applications include photoresiststripping/removal, particle/residue removal for post-chemical mechanicalplanarization (post-CMP cleaning), particle/residue removal forpost-dielectric etching (or post-metal etching), and removal of metalcontaminants.

[0093] A wide variety of contamination-sensitive articles encountered inthe fabrication of microelectronic devices and micro-electromechanicaldevices can be cleaned or processed using embodiments of the presentinvention. Such articles may include, for example, silicon or galliumarsenide wafers, reticles, photomasks, flat panel displays, internalsurfaces of processing chambers, printed circuit boards, surface mountedassemblies, electronic assemblies, sensitive wafer processing systemcomponents, electro-optical, laser and spacecraft hardware, surfacemicro-machined systems, and other related articles subject tocontamination during fabrication. Typical contaminants to be removedfrom these articles in a cleaning process may include, for example, lowand high molecular weight organic contaminants such as exposedphotoresist material, photoresist residue, UV- or X-ray-hardenedphotoresist, C-F-containing polymers and other organic and inorganicetch residues, ionic and neutral, light and heavy inorganic (metal)species, moisture, and insoluble materials, including post-planarizationparticles.

[0094] Dense fluids are well-suited to convey processing agents toarticles such as microelectronic components undergoing processing stepsand for removing undesirable components from the microelectroniccomponents upon completion of the process steps. These process stepstypically are carried out batchwise and may include, for example,cleaning, film stripping, etching, deposition, drying, photoresistdevelopment, and planarization. Other uses for dense fluids includeprecipitation of nano-particles and suspension of metallicnano-crystals.

[0095] Dense fluids are ideal for these applications because thesefluids characteristically have high solvent power, low viscosity, highdiffusivity, and negligible surface tension relative to the articlesbeing processed. As pointed out above, the processing fluids used inmicroelectronic processing must have extremely high purity, much higherthan that of similar fluids used in other applications. The generationof extremely high purity dense fluids for these applications must bedone with great care, preferably using the methods described herein.

[0096] A single-component supercritical fluid is defined as a fluidabove its critical temperature and pressure. A related single-componentfluid having similar properties to a supercritical fluid is asingle-phase fluid which exists at a temperature below its criticaltemperature and a pressure above its liquid saturation pressure. In thepresent disclosure, the term “dense fluid” as applied to asingle-component fluid is defined to include both a supercritical fluidand a single-phase fluid which exists at a temperature below itscritical temperature and a pressure above its saturation pressure. Asingle-component dense fluid also can be defined as a single-phase fluidat a pressure above its critical pressure or a pressure above its liquidsaturation pressure. The term “component” as used herein means anelement (for example, hydrogen, helium, oxygen, nitrogen) or a compound(for example, carbon dioxide, methane, nitrous oxide, sulfurhexafluoride).

[0097] A single-component subcritical fluid is defined as a fluid at atemperature below its critical temperature or a pressure below itscritical pressure.

[0098] A dense fluid alternatively may comprise a mixture of two or morecomponents. In this case, the dense fluid is defined as a single-phasemulti-component fluid of a given composition which is above itssaturation or bubble point pressure, or which has a combination ofpressure and temperature above the mixture critical point. The criticalpoint for a multi-component fluid is defined as the combination ofpressure and temperature above which the fluid of a given compositionexists only as a single phase. In the present disclosure, the term“dense fluid” as applied to a multi-component fluid is defined toinclude both a supercritical fluid and a single-phase fluid that existsat a temperature below its critical temperature and a pressure above itsbubble point or saturation pressure. A multi-component dense fluid alsocan be defined as a single-phase multi-component fluid at a pressureabove its critical pressure or a pressure above its bubble point orliquid saturation pressure. A multi-component dense fluid differs from asingle-component dense fluid in that the liquid saturation pressure,critical pressure, and critical temperature are functions ofcomposition. As described below, dense fluids may be prepared accordingto embodiments of the invention from an initial subcritical fluid havinga fixed density and composition.

[0099] A multi-component subcritical fluid is defined as amulti-component fluid of a given composition which is at or below itssaturation or bubble point pressure, or which has a combination ofpressure and temperature below the mixture critical point.

[0100] The generic definition of a dense fluid thus includes a singlecomponent dense fluid as defined above as well as a multi-componentdense fluid as defined above. Similarly, a subcritical fluid may be asingle-component fluid or a multi-component fluid.

[0101] The definition of a dense fluid for a single component isillustrated in FIG. 1, which is a representative density-temperaturephase diagram for carbon dioxide. This diagram shows saturated liquidcurve 1 and saturated vapor curve 3, which merge at critical point 5 atthe critical temperature of 87.9° F. and critical pressure of 1,071psia. Lines of constant pressure (isobars) are shown, including thecritical isobar of 1,071 psia. Line 7 is the melting curve. The regionto the left of and enclosed by saturated liquid curve 1 and saturatedvapor curve 3 is a two-phase vapor-liquid region. The region outside andto the right of liquid curve 1, saturated vapor curve 3, and meltingcurve 7 is a single-phase fluid region. The dense fluid as definedherein is indicated by cross-hatched region 9.

[0102] A generic density-temperature diagram can be defined in terms ofreduced temperature, reduced pressure, and reduced density as shown inFIG. 2. The reduced temperature (T_(R)) is defined as the absolutetemperature divided by the absolute critical temperature, reducedpressure (P_(R)) is defined as the absolute pressure divided by theabsolute critical pressure, and reduced density (P_(R)) is defined asthe density divided by the critical density. The reduced temperature,reduced pressure, and reduced density are all equal to 1 at the criticalpoint by definition. FIG. 2 shows analogous features to FIG. 1 includingsaturated liquid curve 201 and saturated vapor curve 203, which merge atcritical point 205 at a reduced temperature of 1, a reduced density of1, and a reduced pressure of 1. Lines of constant pressure (isobars) areshown, including critical isobar 207 for which P_(R)=1. The region tothe left of and enclosed by saturated liquid curve 201 and saturatedvapor curve 203 is the two-phase vapor-liquid region. The crosshatchedregion 209 above the P_(R)=1 isobar and to the right of the criticaltemperature T_(R)=1 is a single-phase supercritical fluid region. Thecrosshatched region 211 above saturated liquid curve 201 and to the leftof the critical temperature T_(R)=1 is a single-phase compressed liquidregion. The dense fluid as defined herein includes both single-phasesupercritical fluid region 209 and single-phase compressed liquid region211.

[0103] The generation of a dense fluid in embodiments of the presentinvention is illustrated in FIG. 2. In one embodiment, a saturatedliquid at point a is introduced into a vessel and sealed therein. Thesealed vessel is heated isochorically, i.e., at essentially constantvolume, and isopycnically, i.e., at essentially constant density. Thefluid moves along the line as shown to point a′ to form a supercriticalfluid in region 209. This is generically a dense fluid as defined above.Alternatively, the fluid at point a may be heated to a temperature belowthe critical temperature (T_(R)=1) to form a compressed liquid. Thisalso is a generic dense fluid as defined above. In another embodiment, atwo-phase vapor liquid mixture at point b is introduced into a vesseland sealed therein. The sealed vessel is heated isochorically, i.e., atessentially constant volume, and isopycnically, i.e., at essentiallyconstant density. The fluid moves along the line as shown to point b′ toform a supercritical fluid in region 209. This is generically a densefluid as defined above. In another embodiment, a saturated vapor atpoint c is introduced into a vessel and sealed therein. The sealedvessel is heated isochorically, i.e., at essentially constant volume,and isopycnically, i.e., at essentially constant density. The fluidmoves along the line as shown to point c′ to form a supercritical fluidin region 209. This is generically a dense fluid as defined above.

[0104] The final density of the dense fluid is determined by the volumeof the vessel and the relative amounts of vapor and liquid originallyintroduced into the vessel. A wide range of densities thus is achievableby this method. The terms “essentially constant volume” and “essentiallyconstant density” mean that the density and volume are constant exceptfor negligibly small changes to the volume of the vessel that may occurwhen the vessel is heated.

[0105] A dense fluid for practical application in the present inventionmay be either a single-component fluid or a multi-component fluid, andmay have a reduced temperature in the range of about 0.8 to about 1.8.The reduced temperature is defined here as the absolute temperature ofthe fluid divided by the absolute critical temperature of the fluid.

[0106] The dense fluid may comprise, but is not limited to, one or morecomponents selected from the group consisting of carbon dioxide,nitrogen, methane, oxygen, ozone, argon, hydrogen, helium, ammonia,nitrous oxide, hydrocarbons having 2 to 6 carbon atoms, hydrogenfluoride, hydrogen chloride, sulfur trioxide, sulfur hexafluoride,nitrogen trifluoride, chlorine trifluoride, monofluoromethane,difluoromethane, trifluoromethane, trifluoroethane, tetrafluoroethane,pentafluoroethane, perfluoropropane, pentafluoropropane,hexafluoroethane, hexafluoropropylene, hexafluorobutadiene,octafluorocyclobutane and tetrafluorochloroethane.

[0107] A dense processing fluid is defined as a dense fluid to which oneor more processing agents have been added. A processing agent is definedas a compound or combination of compounds that promote physical and/orchemical changes to an article or substrate in contact with the denseprocessing fluid. These processing agents may include, for example, filmstrippers, cleaning or drying agents, entrainers, etching orplanarization reactants, photoresist developers, and depositionmaterials or reactants. The total concentration of these processingagents in the dense processing fluid typically is less that about 50 wt% and may be in the range of 0.1 to 20 wt %. The dense processing fluidtypically remains a single phase after a processing agent is added to adense fluid. Alternatively, the dense processing fluid may be anemulsion or suspension containing a second suspended or dispersed phasecontaining the processing agent. The dense processing fluid may be usedin processing such as film stripping, cleaning, drying, etching,planarization, deposition, extraction, photoresist development, orformation of suspended nano-particles and nano-crystals.

[0108] The term “processing” or “processed” as used herein meanscontacting an article with a dense fluid or a dense processing fluid toeffect physical and/or chemical changes to the article. The term“article” as used herein means any article of manufacture which can becontacted with a dense fluid or a dense processing fluid. Representativearticles may include, for example, silicon or gallium arsenide wafers;reticles; photomasks; flat panel displays; internal surfaces ofprocessing chambers; printed circuit boards; surface mounted assemblies;electronic assemblies; sensitive wafer processing system components;electro-optical, laser and spacecraft hardware; surface micro-machinedsystems; and other related articles subject to contamination duringfabrication. The term “processing” may include, for example, filmstripping, cleaning, drying, etching, planarization, deposition,extraction, photoresist development, or formation of suspendednano-particles and nano-crystals.

[0109] An embodiment of the invention can be illustrated by thegeneration and use of a dense processing fluid for use in the cleaningor processing of an article such as a microelectronic component. Anexemplary process for this embodiment is shown in FIG. 3, whichillustrates an isochoric (constant volume) carbon dioxide pressurizationsystem to generate a carbon dioxide dense fluid for an ultrasonicelectronic component cleaning chamber or processing tool, and includes acarbon dioxide recovery system to recycle carbon dioxide afterseparation of extracted contaminants. Liquid carbon dioxide and itsequilibrium vapor are stored in carbon dioxide supply vessel 301,typically at ambient temperature; at 70° F., for example, the vaporpressure of carbon dioxide is 854 psia. At least one carbon dioxidepressurization vessel is located downstream of the supply vessel 301. Inthis embodiment, three pressurization vessels 303, 305, and 309(described in more detail below) are shown in flow communication withcarbon dioxide supply vessel 301 via manifold 311 and lines 313, 315,and 317 respectively. These lines are fitted with valves 319, 321, and323, respectively, to control flow of carbon dioxide from supply vessel301 to the pressurization vessels. Fluid supply lines 325, 327, and 329are connected to manifold 331 via valves 333, 335, and 337 respectively.

[0110] A detailed illustration of pressurization vessel 303 is given inFIG. 4. Pressurization vessel 303 comprises outer pressure casing 401,inner vessel 403, and thermal insulation 405 between the inner vesseland the outer pressure casing. The thermal mass of inner vessel 403 ispreferably minimized to minimize the cool-down time when the vessel isinitially filled from carbon dioxide supply vessel 301. Inner vessel 403is in fluid communication with thermal insulation 405 via opening 407 toensure that the pressures inside and outside of inner vessel 403 areapproximately equal, which allows the wall thickness and thermal mass ofinner vessel 403 to be minimized. Opening 407 may contain a de-mistingmedium, such as metal mesh or porous sintered metal (not shown), toprevent liquid carbon dioxide droplets from migrating into thermalinsulation 405.

[0111] The level of liquid in the pressurization vessel may be monitoredconveniently by differential pressure sensor 409, which is in fluidcommunication with the interior of inner vessel 403 via lines 411, 413,and 415. A typical liquid level is shown between liquid 417 and vapor419 in inner vessel 403. Inner vessel 403 is in fluid communication withlines 313 and 325 of FIG. 3 via line 420.

[0112] Heat may be supplied to inner vessel 403 by any desired method.In one embodiment, hot heating fluid 421 is supplied via line 423 toheat exchanger 425, which heats liquid 417 and vapor 419 by indirectheat exchange. Cooled heating fluid is withdrawn via line 427. Heatexchanger 425 can be any type of heat exchange assembly. One type ofuseful heat exchange assembly is a longitudinally-finned pipe as shownin which a plurality of fins 429 are brazed or welded to pipe 431. Thetemperature and flow rate of heating fluid 421 may be regulated tocontrol the heating rate during pressurization and the final temperatureand pressure of the dense fluid formed within inner vessel 403.

[0113] Returning now to FIG. 3, carbon dioxide supply vessel 301 isconnected via two-way flow line 339 to carbon dioxide liquefier 341located above the carbon dioxide supply vessel 301. Heat exchanger 343,which may be a plate and fin or other type of heat exchanger such asheat exchanger 425 of FIG. 4, is used to cool the interior of liquefier341. A cooling fluid is supplied via line 330 and may be, for example,cooling water at an ambient temperature of 70° F., which will maintainthe pressure in carbon dioxide supply vessel 301 at the correspondingcarbon dioxide vapor pressure of 854 psia.

[0114] In this illustration, valve 319 is open while valves 321, 323,and 333 are closed. Valve 335 or 337 may be open to supply dense fluidcarbon dioxide to manifold 331 from pressurization vessel 305 or 309,which previously may have been charged with carbon dioxide andpressurized as described below. Liquid carbon dioxide from supply vessel301 flows downward into pressurization vessel 303 via manifold 311,valve 319, and line 313. As the liquid carbon dioxide enterspressurization vessel 303, which was warmed in a previous cycle, initialliquid flashing will occur. Warm flash vapor returns upward into thecarbon dioxide supply vessel 301 via line 313 and manifold 311 as liquidflows downward into pressurization vessel 303. The warm flash vaporflows back into carbon dioxide supply vessel 301 and increases thepressure therein. Excess vapor flows from supply vessel 301 via line 339to carbon dioxide liquefier 341, wherein the vapor is cooled andcondensed to flow downward via line 339 back to supply vessel 301.

[0115] After initial cooling and pressurization, liquid carbon dioxideflows from supply vessel 301 into pressurization vessel 303. When thepressurization vessel is charged with liquid carbon dioxide to a desireddepth, valve 319 is closed to isolate the vessel. The carbon dioxideisolated in vessel 303 is heated by indirect heat transfer as describedabove and is pressurized as temperature increases. The pressure ismonitored by pressure sensor 345 (pressure sensors 347 and 349 are usedsimilarly for vessels 305 and 309 respectively). As heat is transferredto the carbon dioxide in vessel 303, the temperature and pressure rise,the separate liquid and vapor phases become a single phase, and a densefluid is formed. This dense fluid may be heated further to become asupercritical fluid, which by definition is a fluid at a temperatureabove its critical temperature and a pressure above its criticalpressure. Conversely, the subcritical fluid is defined as a fluid at atemperature below its critical temperature or a pressure below itscritical pressure. The carbon dioxide charged to pressurization vessel303 prior to heating is a subcritical fluid. This subcritical fluid maybe a saturated vapor, a saturated liquid, or a two-phase fluid havingcoexisting vapor and liquid phases.

[0116] As additional heat is transferred, the temperature and pressurequickly rise to supercritical levels to form a supercritical fluidhaving a desired density. The final carbon dioxide pressure in thepressurization vessel of a known volume can be predicted from the volumeof the initial liquid charge. For example, at 854 psia and 70° F. thedensity of liquid carbon dioxide in the vessel is 47.6 lb/ft³ and thedensity of the coexisting carbon dioxide vapor is 13.3 lb/ft³. If theliquid carbon dioxide charge occupies 46.3% of the volume of the vessel,then the carbon dioxide vapor occupies the remaining 53.7% of thevolume. In this example, the average density of all carbon dioxide inthe vessel can be calculated as 0.463 (47.6)+0.537 (13.3), or 29.2lb/ft³.

[0117] Since the internal volume of the vessel and the mass of carbondioxide in the vessel remain essentially unchanged during the heatingstep, the average density of the captured carbon dioxide will remainessentially unchanged at 29.2 lb/ft³ regardless of the temperature andpressure. In this example, heating the selected initial charge of carbondioxide isochorically (at constant volume) at a fixed density of 29.2lb/ft³ will pass through the critical point at the critical temperatureof 87.9° F. and the critical pressure of 1,071 psia. Additional heatingwill form a supercritical fluid at the desired temperature and pressurehaving a fixed density of 29.2 lb/ft³. Using a smaller initial quantityof liquid carbon dioxide in the vessel will result in a lower densitysupercritical fluid; conversely, using a greater initial quantity ofliquid carbon dioxide in the vessel will result in a higher densitysupercritical fluid. Heating a higher density supercritical fluid to agiven temperature will generate a higher pressure than heating a lowerdensity supercritical fluid to the same temperature.

[0118] The highest theoretically achievable pressure is obtained whenthe pressurization vessel initially is completely filled with liquidcarbon dioxide, leaving no vapor head space in the vessel. For example,the average density of the saturated carbon dioxide liquid in the vesselat 70° F. is 47.6 lb/ft³. Initial heating of the liquid carbon dioxidewill change the saturated liquid into a dense fluid in a region of thephase diagram sometimes termed a compressed liquid or subcooled liquid.As the fluid is heated above the critical temperature of 87.9° F., itbecomes a supercritical fluid by definition. In this example, the carbondioxide may be heated at a constant density of 47.6 lb/ft³ to atemperature of 189° F. to yield a supercritical fluid at a pressure ofapproximately 5,000 psia.

[0119] By using the method illustrated in the above examples, a densefluid can be prepared at any selected density, temperature, andpressure. Only two of these three parameters are independent when thecomposition is fixed; the preferred and most convenient way to prepare adense fluid is to select an initial charge density and composition inthe pressurization vessel and then heat-the charge to a desiredtemperature. Proper selection of the initial charge density andcomposition will yield the desired final pressure.

[0120] When carbon dioxide is used for a single-component denseprocessing fluid, the carbon dioxide may be heated to a temperaturebetween about 100° F. and about 500° F. to generate the desired densefluid pressure in the pressurization vessel. More generally, when usingany component or components for the dense fluid, the fluid may be heatedto a reduced temperature in the pressurization vessel of up to about1.8, wherein the reduced temperature is defined as the average absolutetemperature of the fluid in the pressurization vessel after heatingdivided by the absolute critical temperature of the fluid. The criticaltemperature is defined for a fluid containing any number of componentsas that temperature above which the fluid always exists as a singlefluid phase and below which two phases may form.

[0121] Returning now to FIG. 3, valve 333 is opened and dense fluidprepared as described above passes through manifold 331 under flowcontrol through metering valve 351. Optionally, one or more entrainersor processing agents from entrainer storage vessels 353 and 355 may beintroduced by pumps 357 and 359 into the dense fluid in line 361 toprovide a dense processing fluid, which in a cleaning application may bedescribed as a dense cleaning fluid. The dense processing fluid isintroduced into sealable processing chamber or process tool 362 whichholds one or more articles 363 to be cleaned or processed, and valve 333is closed. These articles were previously placed on holder 365 inprocess tool 362 via a sealable entry port (not shown). The temperaturein process tool 362 is controlled by means of temperature control system367. Fluid agitator system 369 mixes the interior of process tool 362 topromote contact of the dense processing fluid with articles 363.

[0122] Processing chamber or process tool 362 is fitted with ultrasonicgenerator 370, which is an ultrasonic transducer array connected to highfrequency power supply 371. The ultrasonic transducer may be anycommercially available unit such as, for example, an ultrasonic hornfrom Morgan Electro Ceramics of Southampton, England. Ultrasonicgenerator 370 typically may be operated in a frequency range of 20 KHzto 2 MHz. In the present disclosure, the term “ultrasonic” refers to anywave or vibration having a frequency above the human audible limit ofabout 20 KHz. High frequency power supply 371 typically provides powerin an ultrasonic power density range of about 20 W/in² to about 40W/in². The interior of process tool 362 typically is exposed toultrasonic waves for 30 to 120 seconds during the cleaning step.

[0123] Ultrasonic transducers can be constructed from piezoelectric ormagnetostrictive structures. Piezoelectric transducers contain crystalsthat oscillate at ultrasonic frequencies when alternating current isapplied. Sturdier magnetostrictive transducers consist of a piece ofiron or nickel surrounded by an electric coil. Such transducers arecommonly built into a “probe” assembly, which includes an acousticallydesigned booster and a horn (not shown). Such probes can be used tomaximize power transmission into the fluid, pressure vessel wall, orsubstrate holder.

[0124] Ultrasonic transducer array 370 may be mounted horizontally aboveand facing the articles being processed, as is shown in FIG. 3, suchthat the sonic waves are generated in a downward direction and impingedirectly on articles 363. Alternatively, the transducer array may bemounted vertically on either side of the articles being cleaned (notshown) such that the ultrasonic waves are generated in a horizontaldirection across the articles being cleaned.

[0125] In another alternative, the transducer array may be mountedhorizontally below and in contact with holder 365 (not shown) such thatthe ultrasonic waves are generated in a generally vertical direction andtransmitted upward through holder 365. This configuration can be used,for example, to apply the maximum ultrasonic energy to the surface of awafer, principally when chemical reactions such as thin film deposition,etching, or electropolishing occur at the wafer surface. The wafer canbe positioned in any orientation, i.e., facing up, facing down, orfacing sideways. In this case, acoustic streaming carries reactionproducts and contaminants away from the surface. The flow sweeps in fromthe sides and away from the surface. Dissolved materials and suspendedparticles tend to migrate away from areas of sonic energy concentration,and this arrangement would tend to carry concentrated materials awayfrom the surface and away from the ultrasonic source.

[0126] While ultrasonic transducer array ultrasonic generator 370 asshown in FIG. 3 is mounted within process tool 362, it may be mountedalternatively on the outside surface of the process tool vessel suchthat the generated sonic energy is transmitted through the walls of thevessel.

[0127] The initial pressure in pressurization vessel 303 and thetemperature in process tool 362 may be selected so that the densecleaning fluid in process tool 362 after the transfer step typically isa single-phase dense fluid as defined above, whether or not an entraineror other processing agent is added to the original dense fluid.Alternatively, the dense processing fluid may be an emulsion orsuspension containing a second suspended or dispersed phase containingthe processing agent.

[0128] A wide variety of contamination-sensitive articles may beencountered in the fabrication of microelectronic devices, and thesemicro-electromechanical devices can be cleaned or processed using thepresent invention. Such articles may include, for example, silicon orgallium arsenide wafers, reticles, photomasks, flat panel displays,internal surfaces of processing chambers, printed circuit boards,surface mounted assemblies, electronic assemblies, sensitive waferprocessing system components, electro-optical, laser and spacecrafthardware, surface micro-machined systems, and other related articlessubject to contamination during fabrication. Typical contaminants thatcan be removed from these articles in a cleaning process may include,for example, low and high molecular weight organic contaminants such asexposed photoresist material, photoresist residue, UV- or X-ray-hardenedphotoresist, C-F-containing polymers and other organic and inorganicetch residues, ionic and non-ionic inorganic metal-containing compounds,moisture, and insoluble materials including post-planarizationparticles.

[0129] Sealed process tool 362 may be pressurized with the densecleaning fluid to a typical supercritical pressure of 1,100 to 10,000psia, preferably 1,500 to 7,500 psia. The tool typically operates at asupercritical temperature of up to 500° F., and may operate in a rangeof 100° F. to 200° F. The temperature in process tool 362 is controlledby means of temperature control system 367. Typically, the contacting ofarticles 363 with the dense cleaning fluid in process tool 362 may beeffected at a reduced temperature above 1.0 and typically below about1.8, wherein the reduced temperature is defined as the average absolutetemperature of the fluid in the cleaning chamber divided by the absolutecritical temperature of the fluid.

[0130] Several alternatives to the introduction of entrainer orprocessing agent into line 361 to mix with the dense fluid prior toflowing into process tool 362 are possible. In one alternative,entrainer may be introduced directly into process tool 362 before thetool is charged with dense fluid from pressurization vessel 303. Inanother alternative, entrainer may be introduced directly into processtool 362 after the tool is charged with dense fluid. In yet anotheralternative, entrainer may be introduced directly into pressurizationvessel 303 before the vessel is charged from supply vessel 301. In afurther alternative, entrainer may be introduced directly intopressurization vessel 303 after the vessel is charged from supply vessel301 but before the vessel is heated. In a final alternative, entrainermay be introduced directly into pressurization vessel 303 after thevessel is charged from supply vessel 301 and after the vessel is heated.Any of these alternatives can be accomplished using the appropriatelines, manifolds, and valves in FIG. 3.

[0131] In addition to the intense agitation provided by ultrasonictransducer 370, the interior of process tool 362 may be mixed by fluidagitator system 369 to enhance contact of the dense cleaning fluid witharticles 363. Additional fluid agitation may be provided by arecirculating fluid system consisting of pump 372 and filter 373. Filter373 serves to remove particulate contamination from the recirculatingfluid, and the resulting fluid agitation mixes the dense fluid andpromotes removal of contaminants or reaction products from thecontaminated articles by increasing convective fluid motion.

[0132] When the cleaning cycle is complete, process tool 362 isdepressurized by opening valves 375 and 377 whereby the contaminateddense fluid flows through heat exchanger 379, where it is cooled to atemperature of 70° F. to 150° F. This reduction in pressure andtemperature condenses the dissolved contaminants and entrainers in thedense fluid, and the resulting fluid containing suspended contaminantsand entrainers flows via line 381 into separator 383. Condensedcontaminants and entrainers are removed via line 385 and the purifiedfluid flows via line 387 to intermediate fluid storage vessel 389. Thepressure in storage vessel 389 is between the supercritical extractionpressure in process tool 362 and the pressure of carbon dioxide supplyvessel 301. Typically, process tool 362 is depressurized in this step toa pressure of 900 to 1,100 psia.

[0133] During the depressurization step, valve 333 optionally may beopened so that carbon dioxide from pressurization vessel 303 also flowsthrough cooler 370 and separator 383 with the contaminateddepressurization fluid. Optionally, after process tool 362 is initiallydepressurized, carbon dioxide from pressurization vessel 303 may be usedto partially pressurize and rinse process tool 362 to dilute and removeresidual contaminants and entrainers therefrom, after which the processtool would be depressurized through cooler 379 and separator 383 to apressure of 900 to 1,100 psia. After closing valves 375 and 377, theremaining carbon dioxide in process tool 362 then is vented throughvalve 391 to reduce the pressure to atmospheric. Process tool 362optionally then may be evacuated to a subatmospheric pressure. At thispoint, the sealable entry port (not shown) of process tool 362 isopened, the processed articles are removed, and another group ofcontaminated articles is loaded for the next cleaning cycle.

[0134] Optionally, another cooler and separator (not shown) similar tocooler 379 and separator 383 may be installed in line 387. The use ofthis second stage of separation at an intermediate pressure allows moreefficient separation of contaminants and entrainers from the carbondioxide solvent, and may allow a degree of fractionation between thecontaminants and entrainers.

[0135] Carbon dioxide in intermediate fluid storage vessel 389,typically at a pressure in the range of 900 to 1,100 psia, may befiltered by filter system 393 before being recycled via line 395 andvalve 397 to liquefier 341, where it is liquefied and returned to carbondioxide supply vessel 301 for reuse. Makeup carbon dioxide may be addedas a vapor through line 398 and valve 399 or added as a liquid directly(not shown) to carbon dioxide supply vessel 301.

[0136] Alternatively, the purified carbon dioxide in line 387 or line395 may be vented directly to the atmosphere (not shown) withoutrecycling as described above. In this embodiment, the carbon dioxide isintroduced via line 398 and valve 399 and is used in a once-throughmode.

[0137] Multiple pressurization vessels may be used in the exemplaryprocess as described above. For example, when pressurization vessel 303of FIG. 3 is in the process of filling and heating, pressurizationvessel 305 (which was previously filled and heated to provide densefluid at the desired conditions) can supply process tool 362 via line327, valve 335, manifold 331, and line 361. A cycle can be envisioned inwhich the three pressurization vessels 303, 305, and 307 operate in astaggered cycle in which one supplies dense fluid to process tool 362,another is being filled with carbon dioxide from carbon dioxide supplyvessel 301, and the third is being heated after filling. Utilizingmultiple pressurization vessels in this manner increases theproductivity of process tool 362 and allows for backup if one of thepressurization vessels is taken off line for maintenance.

[0138] The exemplary process described above uses carbon dioxide as thedense fluid, but other dense fluid components may be used forappropriate applications. The dense fluid may comprise one or morecomponents selected from the group consisting of carbon dioxide,nitrogen, methane, oxygen, ozone, argon, hydrogen, helium, ammonia,nitrous oxide, hydrocarbons having 2 to 6 carbon atoms, hydrogenfluoride, hydrogen chloride, sulfur trioxide, sulfur hexafluoride,nitrogen trifluoride, chlorine trifluoride, and fluorocarbons such as,but not limited to, monofluoromethane, difluoromethane,trifluoromethane, trifluoroethane, tetrafluoroethane, pentafluoroethane,perfluoropropane, pentafluoropropane, hexafluoroethane,hexafluoropropylene (C₃F₆), hexafluorobutadiene (C₄F₆),octafluorocyclobutane (C₄F₈) and tetrafluorochloroethane.

[0139] A dense processing fluid is generically defined as a dense fluidto which one or more processing agents have been added. A processingagent is defined as a compound or combination of compounds that promotephysical and/or chemical changes to an article or substrate in contactwith the dense processing fluid. These processing agents may include,for example, film strippers, cleaning or drying agents, entrainers,etching or planarization reactants, photoresist developers, anddeposition materials or reactants. The total concentration of theseprocessing agents in the dense processing fluid typically is less thatabout 50 wt % and may be in the range of 0.1 to 20 wt %. The denseprocessing fluid typically remains a single phase after a processingagent is added to a dense fluid. Alternatively, the dense processingfluid may be an emulsion or suspension containing a second suspended ordispersed phase containing the processing agent.

[0140] The exemplary process described above with reference to FIG. 3may utilize one or more entrainers mixed with a dense fluid to provide adense film stripping or cleaning fluid containing 0.1 to 20 wt %entrainer. An entrainer is defined as a processing agent that enhancesthe cleaning ability of the dense fluid to remove contaminants from acontaminated article. Entrainers generally may include solvents,surfactants, chelators and chemical modifiers. Some examples ofrepresentative entrainers include acetylenic alcohols and diols,organosilicones, ethyl acetate, ethyl lactate, propyl acetate, butylacetate, diethyl ether, dipropyl ether, methanol, ethanol, isopropanol,acetonitrile, propionitrile, benzonitrile, ethylene cyanohydrin,ethylene glycol, propylene glycol, ethylene glycol monoacetate,propylene glycol monoacetate, acetone, butanone, acetophenone,trifluoroacetophenone, triethyl amine, tripropyl amine, tributyl amine,2,4, dimethyl pyridine, dimethylethanolamine, diethylethanolamine,diethylmethanolamine, dimethylmethanolamine, dimethylformamide,dimethylacetamide, ethylene carbonate, propylene carbonate, acetic acid,lactic acid, butane-diol, propane-diol, n-hexane, n-butane, hydrogenperoxide, t-butyl hydroperoxide, and chelating agents such asethylenediaminetetraacetic acid (EDTA), catechol, choline, beta-diketoneand beta-ketoimine ligands, trifluoroacetic anhydride (TFM), halogenatedcarboxylic acids, halogenated glycols, and halogenated alkanes.

[0141] Dense processing fluids prepared and managed by the methods ofthe present invention may be used in other processing steps in themanufacture of electronic components in which material is removed from apart (etching, drying, or planarization), in which material is depositedon a part (thin film deposition), or in which material on a part ischemically modified (photoresist development).

[0142] Surface etching is a chemical reaction process, typicallyperformed using liquid mixtures or dry plasma processes. Duringsemiconductor substrate processing, such etching is used to reducesurface thickness, remove unwanted layers such as surface oxide, andcreate surface features such as trenches and via holes. Surface etchingcan be performed in a dense phase fluid system using ultrasonic waveenhancement.

[0143] Ultrasonic waves can be used to enhance the reaction speed ofthin metal film deposition. Such films typically are deposited frommetallic precursors that undergo a reduction reaction at a heatedsurface using a reductant such as hydrogen. The use of ultrasonic energywith a dense processing fluid increases the rate of the reaction,thereby improving process efficiency and improving the quality of thethin film.

[0144] Photoresist development is normally performed in a liquid phasesystem using chemicals such as tetramethyl ammonium hydroxide (TMAH) todevelop exposed photoresist. This process can be performed in a densephase fluid system according to the present invention using ultrasonicenergy to enhance the surface chemical reactions which occur inphotoresist development. The application of ultrasonic energy in a densephase processing fluid can improve the diffusion of chemical reactantsand reaction products near the surface of the articles being processed.

[0145] In these alternative processing steps, appropriate processingagents or reactive compounds may be added to the dense fluid to form adense processing fluid. Some representative reactive compounds that canbe added to a dense fluid as processing agents for etching orplanarization processes may include, for example, hydrogen fluoride,hydrogen chloride, hexafluoroethane, nitrogen trifluoride, reactivepolishing slurries (containing alumina, silica, ceria or magnesiumabrasive particles suspended in an acidic, or alkaline, i.e., potassiumhydroxide- or ammonia-containing mixture), and electrolytic solutionsfor reverse electroplating of metal surfaces. Some representativereactive and non-reactive compounds that can be added to a dense fluidfor deposition processes may include, for example include organometallicprecursors, photoresists, photoresist developers, interlayer dielectricmaterials, silane reagents and various coating materials, including butnot limited to stain resistant coatings. A representative reactivecompound that may be added to a dense fluid for photoresist developmentprocessesis tetramethyl ammonium hydroxide (TMAH). Methanol is arepresentative compound that may be added to a dense fluid for dryingprocesses. In these alternative uses of dense processing fluids, processtool 362 of FIG. 3 as described above for cleaning can be replaced withthe appropriate process tool for these alternative applications.

[0146] The present invention combines ultrasonic energy and dense fluidimmersion concurrently in the same processing tool. The semiconductorsubstrate or article being processed thereby is exposed to enhanceddense fluid processing comprising dissolution and/or chemical reactioncombined with a simultaneous, ultrasonic energy enhancement of theprocess. The auxiliary mechanisms for fluid agitation described above,namely fluid agitator system 369 and recirculating fluid systemconsisting of pump 372 and filter 373, may also be used to enhanceultrasonic agitation. In this manner, the solvent or processing agent ina cleaning process can achieve greater penetration into relatively thickcontaminant films, such as photoresist, and the inert insoluble residuesare removed through the energy imposed by fluid phase oscillations.Insoluble particles may be removed through a combination of oscillatoryeffects and acoustic streaming (induced flow in the cleaning fluid).

[0147] The increased rate of solvent penetration into contaminant filmsprovided by ultrasonic energy is advantageous in semiconductor substratecleaning applications, where high throughput is necessary in order toprovide an economical process. Ultrasonic agitation also tends toincrease the uniformity of the cleaning process and thereby providesbetter cleaning or surface treatment performance than can be achievedusing dense fluid immersion alone.

[0148] Ultrasonic energy causes localized pressure fluctuations in thedense fluid or dense processing fluid, which aids in cleaning orprocessing performance. These pulsations in pressure created by theultrasonic waves cause corresponding oscillations in the density of thedense fluid about a mean value, which in turn causes correspondingoscillations in the solvent power of the fluid about the mean value. Thesolvent power therefore varies cyclically between maximum and minimumvalues during the process, and maximum achieved solvent power thereforeexceeds the mean value that would be available without ultrasonic waves.This in turn increases the overall effectiveness of the dissolutionprocess without a concomitant increase in the mean operating pressure.Conventional wet ultrasonic cleaning utilizes transient cavitation ofliquids followed by bubble collapse, and the resulting energy release,to dislodge contaminants. Such cavitation can damage the delicatefeatures of modem semiconductor devices. The process of the presentinvention preferably is operated entirely in the dense fluid region suchthat no phase change occurs, and therefore no cavitation can occur.Embodiments of the invention instead utilize high frequency fluidoscillations to excite adhered contaminants near their naturalfrequencies, thereby producing dislodgement. Since cavitation issuppressed, power dissipation is minimized, and acoustic streaming isenhanced.

[0149] A further embodiment of the invention is the use of variablefrequency ultrasonic treatment in which the ultrasonic frequency isvaried during the processing period. Variable frequency ultrasonictreatment eliminates static vibrational nodes on the surface of thearticle being processed and ensures, for example, that particles havingwide-ranging natural frequencies are dislodged and suspended in thedense processing fluid. Frequencies utilized in this invention may spanthe range from typical ultrasonic to megasonic values (approximately 20KHz to 2 MHz). In one embodiment, the variable frequency ultrasonictreatment may comprise starting the cleaning or processing period at afrequency in the lower end of this range and increasing the frequencygradually during the cleaning period to a final frequency in the upperend of this range. Alternatively, the variable frequency ultrasonictreatment may comprise starting the cleaning or processing period at afrequency in the higher end of this range and decreasing the frequencygradually during the period to a final frequency in the lower end ofthis range. In another alternative, the frequency can be raised andlowered in this range multiple times during the cleaning or processingperiod.

[0150] In another embodiment of the invention, ultrasonic energy isintroduced intermittently into the process tool during the cleaning orprocessing period. In this embodiment, on-off actuation of theultrasonic transducer provides intermittent bursts of power in the densephase processing fluid. Such pulsing, for example, would preventcontaminants from being trapped in standing waves during a cleaningstep. The frequency may be either constant or variable when theultrasonic transducer is on, and the frequency may be either constant orvariable among multiple pulse periods.

[0151] Variable frequency or intermittent ultrasonic treatment may beused with dense fluids that contain no processing agents or with denseprocessing fluids that by definition contain one or more processingagents. Any combination or schedule of frequency changes and/orintermittent treatment periods during the cleaning or processing stepmay be used with dense fluids or with dense processing fluids. Theapplication of variable frequency and/or intermittent ultrasonic energyis particularly useful in combination with selected processing agents indense processing fluids for the removal of various types of contaminantparticles from the articles being cleaned.

[0152] The use of ultrasonic energy complements dense fluid cleaning orprocessing since dissolution is more effective for smaller, solubleparticles while ultrasonic cleaning or processing is more effective forlarger or insoluble particles. Ultrasonic cleaning or processing workswell in deeply patterned surfaces, i.e., it is not topography sensitive,and the method is adaptable to automation. Ultrasonic dense fluidcleaning can provide comparable performance to wet megasonic cleaningbut without the limitations of wet chemical processing. For example,either method can provide 90% removal of particles having diameters of0.5 micrometer and larger, resulting in a surface density of less than0.1 particle/cm².

[0153] The dense fluid or dense processing fluid used with variablefrequency and/or intermittent ultrasonic treatment may be provided bythe methods described earlier with reference to FIG. 3. Alternatively,the dense fluid or dense processing fluid for use with variablefrequency and/or intermittent ultrasonic treatment may be prepareddirectly in the processing vessel by introducing a subcritical fluidinto the sealable processing chamber and isolating the chamber, heatingthe subcritical fluid at essentially constant volume and essentiallyconstant density to yield a dense fluid, and providing the denseprocessing fluid by one or more steps selected from the group consistingof

[0154] (1) introducing one or more processing agents into the sealableprocessing chamber before introducing the subcritical fluid into thesealable processing chamber,

[0155] (2) introducing one or more processing agents into the sealableprocessing chamber after introducing the subcritical fluid into thesealable processing chamber but before heating the subcritical fluidtherein, and

[0156] (3) introducing one or more processing agents into the sealableprocessing chamber after introducing the subcritical fluid into thesealable processing chamber and after heating the subcritical fluidtherein.

[0157] Dense fluids and dense processing fluids are well-suited forultrasonic processing. The relatively low viscosity of these fluidstends to minimize the rate of viscous dissipation of ultrasonic waves inthe fluid. Therefore, the ultrasonic waves can be delivered to thesurface being processed with relatively little reduction in intensity.This permits high process efficiency at minimal power consumption. Lowviscous dissipation also tends to increase acoustic streaming in thedense fluid or dense processing fluid in cleaning processes, therebypromoting the removal of particulate and dissolved contaminants from thevicinity of the surface through a flushing action. This tends to bringfresh solvent into close proximity to the surface, thereby creating ahigher concentration gradient for dissolved contaminants near thesurface and increasing the rate of diffusion of dissolved contaminantsaway from the surface. The result is a reduction in the processing timerequired to produce a clean surface.

[0158] The relatively low viscosity of dense fluids also helps to reducethe thickness of the fluid boundary layer near the surface. The tendencytoward thinner acoustic boundary layers can be seen from the followingequation for acoustic boundary layer thickness, δ_(ac):$\delta_{a\quad c} = \sqrt{\frac{v}{\pi \quad f}}$

[0159] where ν is the kinematic viscosity of the fluid and f is thefrequency of the waves. A thinner fluid boundary layer tends to promoteremoval of adhered surface particles, since they are exposed to agreater average fluid velocity in a thin boundary layer than if theywere shielded in a thicker, low velocity boundary layer.

[0160] In applying the present invention, semiconductor substrates maybe cleaned or processed individually in order to provide direct processintegration with other, single substrate processing modules.Alternatively, multiple substrates, or batches, may be cleaned orprocessed simultaneously in a container or “boat” placed within thecleaning or processing chamber, thereby providing high throughput andreduced cost of operation.

[0161] Ultrasonic waves improve semiconductor substrate cleaning throughdense fluid immersion by providing a method for removing insolublecontaminants from surfaces using fluid oscillations and acousticstreaming and increasing the rate of penetration of solvents andco-solvents into thick film contaminant layers. As a result, therequired processing time can be reduced. The thickness of theconcentration boundary layer of dissolved reactants or contaminants maybe decreased near the surface by acoustic streaming. This increases thediffusion rate of dissolved reactants to the surface or contaminantsaway from the surface, thereby reducing the required processing time.This also reduces the required density of the dense processing fluidnecessary to achieve effective dissolution of soluble reactants orcontaminants. This in turn reduces the required pressure of the denseprocessing fluid and reduces the overall cost of processing equipmentnecessary to achieve effective processing conditions. Also, this reducesthe amount of dense fluid necessary to achieve effective processingperformance and reduces the required concentrations and amounts ofentrainers or reactants necessary to achieve effective processingperformance in a dense processing fluid. As a result, the overall costof ownership of the process, chemical disposal requirements, energyrequirements, and environmental damage caused by the process can bereduced.

[0162] The following Examples illustrate embodiments of the presentinvention but do not limit the embodiments to any of the specificdetails described therein.

EXAMPLE 1

[0163] An embodiment of the invention according to FIG. 3 is used totreat a silicon wafer having a photoresist layer that has undergonemultiple processing steps including exposure, development, etchingand/or implantation with a dense processing fluid as described below.

[0164] Step 1: Pressurization vessel 303 having a volume of 2.71 litersis filled completely with 4.56 lb of saturated liquid CO₂ at 70° F. and853.5 psia. The density of the initial CO₂ charge is 47.6 lb/ft³. Thevessel is sealed.

[0165] Step 2: The pressurization vessel is heated until the internalpressure reaches 5,000 psia. The density of the contained CO₂ remains at47.6 lb/ft³, and the temperature reaches 189° F. The contained CO₂ isconverted to a dense fluid in the supercritical region (see FIG. 1).

[0166] Step 3: A contaminated silicon wafer is loaded into process tool362 having an interior volume of 1 liter. The process tool is evacuatedand the vessel walls and wafer are held at 104° F.

[0167] Step 4: Valve 333 connecting pressurization vessel 303 viamanifold 331 and line 361 to the process tool 362 is opened, CO₂ flowsfrom pressurization vessel 303 into process tool 362, and the wafer isimmersed in dense phase CO₂. The temperature of pressurization vessel303 remains at 189° F. The common pressure of the pressurization vesseland process module is 2,500 psia. The temperature of the process tool,362, remains at 104° F. The dense phase CO₂ remains in the supercriticalstate in both vessels as 1.79 lb of CO₂ flows into 1 liter process tool362 while the remaining 2.77 lb of CO₂ remains in 2.71 literpressurization vessel 303. The density of the CO₂ in the cooler processtool reaches 50.6 lb/ft³.

[0168] Step 5: An entrainer, propylene carbonate, is pumped fromentrainer storage vessel 353 by pump 357 into process tool 362 and theprocess tool is isolated. The concentration of propylene carbonate inthe dense fluid in the process tool is 1 wt %. The dense fluid isagitated in process tool 362 for two minutes, during which time thewafer is processed to remove contaminants. Ultrasonic transducer 370 isoperated during this period at an ultrasonic frequency of 40 KHz whilehigh frequency power supply 371 provides power at an ultrasonic powerdensity of 40 W/in².

[0169] Step 6: Valves 333, 351, 375, 377, and 397 are opened so thatfluid in process tool 362 and pressurization vessel 303 flows throughcooler 379 and phase separator 383 to carbon dioxide liquefier 341 whilethe pressure in the system is held at 900 psia. Entrainers, reactionproducts, and contaminants are separated from the CO₂ in the separator383. The temperature of pressurization vessel 303 remains at 189° F.during this step and the temperature of the process tool remains at 104°F. during this step. CO₂ is in the vapor phase in both vessels.Neglecting the relatively small effect of other mixture constituents,the density of CO₂ in process tool 362 is 10.32 lb/ft³. 0.36 lb of CO₂remains in the process tool 362.

[0170] Step 7: Pressurization vessel 303 is isolated by closing valve333 and the vessel is cooled to 70° F., wherein the pressure falls to632 psia, and the density of the contained CO₂ vapor in the vesselremains at 7.07 lb/ft³.

[0171] Step 8: The remaining 0.36 lb of CO₂ in the process tool 362 isvented by closing valve 375 and opening valve 391, the tool isevacuated, and the clean, processed silicon wafer is removed.

[0172] The cycle is repeated by returning pressurization vessel 303 toStep 1 by refilling with liquid CO₂.

EXAMPLE 2

[0173] The process of Example 1 is repeated except that ultrasonictransducer system 370 is operated during the cleaning period of step (5)at a sonic frequency which begins at 20 KHz and is increased at aconstant rate during the cleaning period such that the sonic frequencyis 200 KHz at the end of the cleaning period.

EXAMPLE 3

[0174] The process of Example 1 is repeated except that ultrasonictransducer 370 is operated during the cleaning period of step (5) at asonic frequency which starts at 200 KHz and is decreased at a constantrate during the cleaning period such that the sonic frequency is 20 KHzat the end of the cleaning period.

EXAMPLE 4

[0175] The process of Example 1 is repeated except that ultrasonictransducer 370 is operated intermittently by turning the transducersystem on for 1 second and off for 1 second in an alternating patternduring the cleaning period of step (5). The sonic frequency is 40 KHzduring the time the transducer system is on.

1. A method for processing an article comprising: (a) introducing thearticle into a sealable processing chamber and sealing the processingchamber; (b) preparing a dense fluid by: (b1) introducing a subcriticalfluid into a pressurization vessel and isolating the vessel; and (b2)heating the subcritical fluid at essentially constant volume andessentially constant density to yield a dense fluid; (c) transferring atleast a portion of the dense fluid from the pressurization vessel to theprocessing chamber, wherein the transfer of the dense fluid is driven bythe difference between the pressure in the pressurization vessel and thepressure in the processing chamber, thereby pressurizing the processingchamber with transferred dense fluid; (d) introducing one or moreprocessing agents into the processing chamber either before (c), orduring (c), or after (c) to provide a dense processing fluid; (e)introducing ultrasonic energy into the processing chamber and contactingthe article with the dense processing fluid to yield a spent denseprocessing fluid and a treated article; and (f) separating the spentdense processing fluid from the treated article.
 2. The method of claim1 wherein the dense fluid is generated in (b2) at a reduced temperaturein the pressurization vessel below about 1.8, wherein the reducedtemperature is defined as the average absolute temperature of the densefluid in the pressurization vessel after heating divided by the absolutecritical temperature of the fluid.
 3. The method of claim 2 wherein thecontacting of the article with the dense processing fluid in theprocessing chamber in (d) is effected at a reduced temperature in theprocessing chamber between about 0.8 and about 1.8, wherein the reducedtemperature is defined as the average absolute temperature of the denseprocessing fluid in the processing chamber during (d) divided by theabsolute critical temperature of the dense processing fluid.
 4. Themethod of claim 1 wherein the dense fluid comprises one or morecomponents selected from the group consisting of carbon dioxide,nitrogen, methane, oxygen, ozone, argon, hydrogen, helium, ammonia,nitrous oxide, hydrogen fluoride, hydrogen chloride, sulfur trioxide,sulfur hexafluoride, nitrogen trifluoride, monofluoromethane,difluoromethane, trifluoromethane, trifluoroethane, tetrafluoroethane,pentafluoroethane, perfluoropropane, pentafluoropropane,hexafluoroethane, hexafluoropropylene, hexafluorobutadiene, andoctafluorocyclobutane and tetrafluorochloroethane.
 5. The method ofclaim 1 wherein the dense fluid comprises one or more hydrocarbonshaving 2 to 6 carbon atoms.
 6. The method of claim 1 wherein the totalconcentration of the one or more processing agents in the denseprocessing fluid is between about 0.1 and 20 wt %.
 7. The method ofclaim 1 wherein the dense processing fluid comprises one or moreprocessing agents selected from the group consisting of ethyl acetate,ethyl lactate, propyl acetate, butyl acetate, diethyl ether, dipropylether, methanol, ethanol, isopropanol, acetonitrile, propionitrile,benzonitrile, ethylene cyanohydrin, ethylene glycol, propylene glycol,ethylene glycol monoacetate, propylene glycol monoacetate, acetone,butanone, acetophenone, trifluoroacetophenone, triethyl amine, tripropylamine, tributyl amine, 2,4, dimethyl pyridine, dimethylethanolamine,diethylethanolamine, diethylmethanolamine, dimethylmethanolamine,dimethylformamide, dimethylacetamide, ethylene carbonate, propylenecarbonate, acetic acid, lactic acid, butane-diol, propane-diol,n-hexane, n-butane, hydrogen peroxide, t-butyl hydroperoxide,ethylenediaminetetraacetic acid, catechol, choline, and trifluoroaceticanhydride.
 8. The method of claim 1 wherein the dense processing fluidcomprises one or more processing agents selected from the groupconsisting of hydrogen fluoride, hydrogen chloride, chlorinetrifluoride, nitrogen trifluoride, monofluoromethane, difluoromethane,trifluoromethane, trifluoroethane, tetrafluoroethane, pentafluoroethane,perfluoropropane, pentafluoropropane, hexafluoroethane,hexafluoropropylene, hexafluorobutadiene, octafluorocyclobutanetetrafluorochloroethane, fluoroxytrifluoromethane (CF₄O),bis(difluoroxy)methane (CF₄O₂), cyanuric fluoride (C₃F₃N₃), oxalylfluoride (C₂F₂O₂), nitrosyl fluoride (FNO), carbonyl fluoride (CF₂O),and perfluoromethylamine (CF₅N).
 9. The method of claim 1 wherein thedense processing fluid comprises one or more processing agents selectedfrom the group consisting of organometallic precursors, photoresists,photoresist developers, interlayer dielectric materials, silanereagents, and stain-resistant coatings.
 10. The method of claim 1 whichfurther comprises reducing the pressure of the spent dense processingfluid to yield at least a fluid phase and a residual compound phase, andseparating the phases to yield a purified fluid and recovered residualcompounds.
 11. The method of claim 10 which further comprises recyclingthe purified fluid to provide a portion of the subcritical fluid in(b1).
 12. The method of claim 10 which further comprises reducing thepressure of the purified fluid to yield a further-purified fluid phaseand an additional residual compound phase, and separating the phases toyield a further-purified fluid and additional recovered residualcompounds.
 13. The method of claim 12 which further comprises recyclingthe further-purified fluid to provide a portion of the subcritical fluidin (b1).
 14. The method of claim 1 wherein the subcritical fluid in thepressurization vessel prior to heating in (b2) comprises a vapor phase,a liquid phase, or coexisting vapor and liquid phases.
 15. A method forprocessing an article comprising: (a) introducing the article into asealable processing chamber and sealing the processing chamber; (b)preparing a dense processing fluid by: (b1) introducing a subcriticalfluid into a pressurization vessel and isolating the vessel; (b2)heating the subcritical fluid at essentially constant volume andessentially constant density to yield a dense fluid; and (b3)introducing one or more processing agents into the pressurization vesselbefore introducing the subcritical fluid into the pressurization vessel,or after introducing the subcritical fluid into the pressurizationvessel but before heating the pressurization vessel, or afterintroducing the subcritical fluid into the pressurization vessel andafter heating the pressurization vessel; (c) transferring at least aportion of the dense processing fluid from the pressurization vessel tothe processing chamber, wherein the transfer of the dense processingfluid is driven by the difference between the pressure in thepressurization vessel and the pressure in the processing chamber,thereby pressurizing the processing chamber with transferred denseprocessing fluid; (d) introducing ultrasonic energy into the processingchamber and contacting the article with the transferred dense processingfluid to yield a spent dense processing fluid and a treated article; and(e) separating the spent dense processing fluid from the treatedarticle.
 16. An apparatus for processing an article which comprises: (a)a fluid storage tank containing a subcritical fluid; (b) one or morepressurization vessels and piping means for transferring the subcriticalfluid from the fluid storage tank to one or more pressurization vessels;(c) heating means to heat the contents of each of the one or morepressurization vessels at essentially constant volume and essentiallyconstant density to convert the subcritical fluid into a dense fluid;(d) a sealable processing chamber for contacting an article with thedense fluid; (e) ultrasonic generation means for introducing ultrasonicenergy into the sealable processing chamber; (f) piping means fortransferring the dense fluid from the one or more pressurization vesselsinto the sealable processing chamber; and (g) one or more processingagent storage vessels and pumping means to inject one or more processingagents (1) into the one or more pressurization vessels or (2) into thepiping means for transferring the dense fluid from the one or morepressurization vessels to the sealable processing chamber or (3) intothe sealable processing chamber.
 17. The apparatus of claim 16 whichfurther comprises pressure reduction means and phase separation means toseparate a spent dense processing fluid withdrawn from the processingchamber to yield at least a purified fluid and one or more recoveredresidual compounds.
 18. The apparatus of claim 17 which furthercomprises recycle means to recycle the purified fluid to the fluidstorage tank.
 19. A method for processing an article comprising: (a)introducing the article into a sealable processing chamber and sealingthe processing chamber; (b) providing a dense processing fluid in theprocessing chamber; (c) introducing ultrasonic energy into theprocessing chamber and varying the frequency of the ultrasonic energywhile contacting the article with the dense processing fluid to yield aspent dense processing fluid and a treated article; and (e) separatingthe spent dense processing fluid from the treated article.
 20. Themethod of claim 19 wherein the frequency of the ultrasonic energy isincreased during (c).
 21. The method of claim 19 wherein the frequencyof the ultrasonic energy is decreased during (c).
 22. The method ofclaim 19 wherein the dense processing fluid is prepared by: (a)introducing a subcritical fluid into a pressurization vessel andisolating the vessel; (b) heating the subcritical fluid at essentiallyconstant volume and essentially constant density to yield a dense fluid;and (c) providing the dense processing fluid by one or more stepsselected from the group consisting of (1) introducing one or moreprocessing agents into the dense fluid while transferring the densefluid from the pressurization vessel to the processing chamber, (2)introducing one or more processing agents into the pressurization vesselto form a dense processing fluid and transferring the dense processingfluid from the pressurization vessel to the processing chamber, (3)introducing one or more processing agents into the derise fluid in theprocessing chamber after transferring the dense fluid from thepressurization vessel to the processing chamber, (4) introducing one ormore processing agents into the pressurization vessel before introducingthe subcritical fluid into the pressurization vessel, (5) introducingone or more processing agents into the pressurization vessel afterintroducing the subcritical fluid into the pressurization vessel butbefore heating the pressurization vessel, and (6) introducing one ormore processing agents into the pressurization vessel after introducingthe subcritical fluid into the pressurization vessel and after heatingthe pressurization vessel.
 23. The method of claim 19 wherein the denseprocessing fluid is prepared by: (a) introducing a subcritical fluidinto the sealable processing chamber and isolating the chamber; (b)heating the subcritical fluid at essentially constant volume andessentially constant density to yield a dense fluid; and (c) providingthe dense processing fluid by one or more steps selected from the groupconsisting of (1) introducing one or more processing agents into thesealable processing chamber before introducing the subcritical fluidinto the sealable processing chamber, (2) introducing one or moreprocessing agents into the sealable processing chamber after introducingthe subcritical fluid into the sealable processing chamber but beforeheating the subcritical fluid therein, and (3) introducing one or moreprocessing agents into the sealable processing chamber after introducingthe subcritical fluid into the sealable processing chamber and afterheating the subcritical fluid therein.
 24. A method for processing anarticle comprising: (a) introducing the article into a sealableprocessing chamber and sealing the processing chamber; (b) providing adense fluid in the processing chamber; (c) introducing ultrasonic energyinto the processing chamber and varying the frequency of the ultrasonicenergy while contacting the article with the dense fluid to yield aspent dense fluid and a treated article; and (e) separating the spentdense fluid from the treated article.
 25. A method for processing anarticle comprising: (a) introducing the article into a sealableprocessing chamber and sealing the processing chamber; (b) providing adense processing fluid in the processing chamber; (c) introducingultrasonic energy into the processing chamber intermittently whilecontacting the article with the dense processing fluid to yield a spentdense processing fluid and a treated article; and (e) separating thespent dense processing fluid from the treated article.
 26. A method forprocessing an article comprising: (a) introducing the article into asealable processing chamber and sealing the processing chamber; (b)providing a dense fluid in the processing chamber; (c) introducingultrasonic energy into the processing chamber intermittently whilecontacting the article with the dense fluid to yield a spent dense fluidand a treated article; and (e) separating the spent dense fluid from thetreated article.